1. Field of the Invention
The present invention generally relates to the field of radiation sensors. More particularly, the present invention relates to a monolithic integrated focal plane sensor array having elements sensitive to millimeter-wave radiation and elements sensitive to infrared radiation.
2. Description of the Related Art
Radiation sensors are generally known which convert electromagnetic radiation for radiation sensing equipment that is capable of detecting and monitoring electromagnetic radiation of varied wavelength bands to provide a multi-mode sensing type of device. In the prior art, physically separate, non-integrated millimeter-wave (mm-wave or MMW) and infrared (IR) sensing units have been used to provide separate sensing and detecting capabilities in those two different electromagnetic wavelength bands, which, however, leads to a bulky overall system. Several prior attempts have been made to fabricate a single integrated system incorporating two separate FPAs (focal plane arrays), with one being a mm-wave FPA and the other an IR FPA, as provided on a common support body or substrate. However, millimeter-wave radiation of approximately 94 GHz and IR at 3-5 and 8-12 .mu.m (where atmospheric transmission of IR is greatest) differ so significantly in frequencies from each other that integrating these different types of electromagnetic radiation sensors into a single device has posed a technical and practical challenge in the field. Sometimes these integrated systems have used a single aperture for receiving incident radiation at the sensor. The systems separate the incoming radiation energy into mm-wave and IR beams by means of a series of beam splitters to direct the appropriate energy to the appropriate FPA. However, such an approach results in increased cost, lower efficiency and increased size of the overall system.
U.S. Pat. Nos. 4,654,622 and 5,450,053 describe incorporating antennas in microbolometer detector architectures used for IR/mm-wave detection to provide a mm-wave energy coupling apparatus. These patents describe, as one embodiment, a linear sensor array on the front surface of a substrate that combines planar dipole or bowtie type antennas to couple the mm-wave radiation microsensors with interdispersed separate microsensors used for IR sensing. The mm-wave sensors sense only one polarization. The configuration of this linear geometry type of focal plane sensor array in these patents can not be extended to a two-dimensional (2-D) array. Also, the focal plane sensor array can not sense the other dimensional (2-D) array. Also, the focal plane sensor array can not sense the other polarization without losing .apprxeq.10.times. of IR resolution. In another described embodiment in these patents, fabrication of a two-dimensional array is proposed where "bow-tie" microantenna designs are formed on the backside of a silicon substrate while IR sensitive microdetector arrays are formed on the opposite frontside of the substrate, such that incident radiation is collected by the antennas after passing through the substrate. Again, the single "bowtie" microantennas sense only one polarization. This two-dimensional sensor array approach of the above-identified patents is considered problematic because, among other things, incident mm-wave radiation would be reflected by the intervening XY matrix read-out wiring associated with the IR sensor array before it reaches the antennas located on the backside of the substrate.
Consequently, there remains a need for improved monolithic sensors that integrate an IR sensor array with mm-wave sensors on a common support in way that can be conveniently fabricated while resulting in a high performance radiation sensing device.